1. Field of the Invention
The present invention relates to a device for hydraulically adjusting components, in particular, rollers of strand guiding segments of a continuous casting installation, comprising hydraulic cylinders which are separated by a piston with piston rod into a cylinder chamber and an annular cylinder chamber, respectively, wherein the cylinder chambers can be connected by control members alternatingly and in opposite directions as well as simultaneously with a pressure source and a pressure sink.
2. Description of the Related Art
In continuous casting installations the casting process begins in a casting mold. The surficially solidified casting strand exits from the mold vertically and is deflected by 90xc2x0 in strand guiding segments having a certain radius and is guided to a horizontal straightening driver arranged downstream. The guiding of the casting strand is realized by guiding rollers which can be adjusted by hydraulic cylinders. Accordingly, it is possible to consider, inter alia, roll wear and changed casting parameters. For controlling the hydraulic cylinder, in general, proportional servo valves are used. They require microfine-filtered hydraulic oil as a result of their precise fit. The expenditure in regard to oil filtering is significant. Moreover, there is always a fire danger in connection with hydraulic oil in a casting machine and rolling mill area.
The invention has the object to provide a hydraulic system for adjusting continuous strand guiding segments which has a comparatively minimal requirement with regard to the purity of the working liquid and the minimal fire danger.
The object is solved in that as control members on-off valves are provided. On-off valves are either closed or open while proportional servo valves can also be in any intermediate position. Accordingly, on-off valves can withstand coarse dirt particles without clogging, while the possible opening of a proportional servo valve requires a clean working medium in order to prevent seat soiling. Moreover, in contrast to the proportional servo valves, the valve pistons of the on-off valves do not require precise fit because in the open state they are centered on a stop and in the closed state on the seat. Accordingly, the requirement with regard to purity and, moreover, to the lubrication action of the working fluid in the case of on-off valves is significantly lower than in the case of proportional servo valves. This means a reduced expenditure for filtering and the problem-free use of water-oil emulsions as a working liquid. In addition to the reduced expenditure for the on-off valves and for the type as well as the cleaning of the working liquid, the fire safety is a decisive advantage of the solution according to the invention.
By arrangement of four on-off valves in a full bridge circuit, a simple guiding and minimal length of the hydraulic lines with corresponding minimal installation expenditure are achieved.
It is also advantageous that the on-off valves are controllable by a three-step controller. The three-step controller operates only with the positions plus, minus, and zero. In the plus position, one of the on-off valve pairs is excited, in the minus position the other one, while in the zero position both on-off valve pairs are without current and thus closed. This results in a simple control configuration.
Since the on-off valves have a throttle, an adjustment of the piston position without overswinging can be realized despite the fully open on-off valves.
It is also advantageous when the on-off valves can be controlled by pulse width modulation. While in the three-step control the opening time interval of the on-off valves as a whole can be varied, the number of constant, short opening intervals is variable in connection with the pulse width modulation (variable pulse-duty factor). This is realized, similar to the three-step control, by means of discrete switching signals of a separate electronic hardware or by means of software of a computer. This optimizes the pulse-duty factor in the direction of switching frequency reduction. In the pulse width modulation, as in the case of the three-step control, an outlet on-off valve and an inlet on-off valve are always controlled at the same time since the inflow volume of one cylinder chamber corresponds always to the outflow volume of the other one.
It is moreover advantageous that each piston is connected by means of a connecting rod with a position transducer which triggers the control when the piston position surpasses an upper or a lower limit value. The position transducer allows a closed control circuit for the piston position. In this connection, a simple configuration and a simple startup of the three-step controller results based on the determination of hysteresis of the permissible piston position by an upper and lower limit value thereof.
Since the four on-off valves are combined to an on-off valve block, a space-saving, cost-efficient configuration of the on-off valves is provided.